In modern community antenna television (CATV) distribution networks, there have been recent shifts to multi-redundant centralized powering and to the extension of networks by using fiber optic transmission cables. In order to provide these expanded networks and their power hungry equipment with AC power from a single source, the alternating current voltage in these systems is being increased from the 60 volts to 90 volts. The resulting networks tend to have higher voltage drops along the distribution tree than previous networks. These power distribution systems have been plagued by unstable conditions wherein direct current (DC) power supplies begin drawing increased current and overloading the AC power delivery capabilities of the network. The cause of these instabilities is in the nature of modern DC power supplies.
In an ideal electrical system consisting of a power source and a load, maximum power transfer occurs when the impedance of the load matches the impedance of the power supply. If the load resistance increases then current through the system will decrease and less power will be transferred. If the load resistance decreases then the voltage across the load will decrease and less power will be transferred.
In a CATV system, long conductor distances have significant series resistance so that remote portions of these systems tend to have voltages far below the voltage at the AC power source. Optical cables may extend as far as 60 miles from the head end to nodes and then coaxial cables may extend many more miles from the nodes and/or customer interface units. In addition AC voltage in the system may vary dynamically due to momentary power interruptions, sags, system maintenance, adding new users, activation of various equipment in the system, temporary overloads, or "sheath currents" caused by imbalances in the power distribution system.
In CATV networks, at nodes, signal amplifiers, and user interface units, AC power is converted to DC power by a DC power supply, and the DC power is used to operate various electronic equipment. Switched-mode power supplies (SMPSS) are typically used in these applications because they maintain high conversion efficiency over a reasonable range of input voltage, so that power requirements remain nearly constant within that range.
Unfortunately, this constant power characteristic provokes instability in the AC power distribution network because the input impedance of the DC power supplies is dynamically negative. That is, a decrease in the supply voltage causes an increase in the current demanded by the power supplies which causes increased voltage drop through the distribution system and results in a furthers decrease in the voltage at the input of the DC power supply.
The electronic equipment in the nodes and customer interface units are very sensitive to instabilities and to any out of specification conditions in the DC voltage. Such equipment may fail to operate correctly or may even be damaged by instabilities in the DC power.
In modern switched mode power supplies, when the AC input voltage drops below a minimum level, then the DC output becomes unregulated and unstable, and the current demanded by the power supply increases. The increased current further reduces the input voltage, as described above. Thus, once a DC power supply in one branch of the power distribution network becomes unstable, then other DC power supplies in the branch tend to also become unstable resulting in cascade failures. A large number of CATV cable customers are subjected to service interruption. In addition, the recovery voltage at which an unstable DC power supply regains its stability is significantly higher than the voltage at which it initially became unstable, so that it is difficult to regain stability after DC power supplies in a CATV system become unstable.
Currently, the typical response to instability is to shut down the entire system or at least a portion of the system that can be remotely shut down, and then power distribution is reinitiated in the shut down portion. This solution results in interrupted service to a large number of customers.
Those skilled in the art are directed to the following citations. "Powering Stability in 90 Volt Networks" by Peter Deierlein in 1996 NCTA Technical Papers describes the power distribution system instability problem and suggests that "a latching shutdown function (with delayed restart) is added to the power supplies". U.S. Pat. No. 4,937,731 to Konopka suggests a DC power supply that automatically adapts to operate with AC input of either 120 or 240 Volts. U.S. Pat. No. 4,933,832 to Schneider suggests a DC power supply that automatically adapts to operate with an input voltage of either 100 volts or 120 volts. Those skilled in the art are also directed to "Constant power fusing arrangement for 110/220-volt power supplies" and "Primary controlled automatic line select circuit" in IBM technical discolsure bulletin Vol. 31, No. 6, November 1998
The above citations are hereby incorporated herein in whole by reference.